This application claims the priority of German application 198 59 616.2, filed in Germany on Dec. 23, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a brake unit with a nonmetal friction ring made of a ceramic material that is connected directly to a hub, said hub being made in the form of a sleeve which includes a steering knuckle and to which a wheel is fastened, and with an outer edge of the hub radially overlapping an inner edge of the friction ring and having fastening elements in this overlapping area to connect the friction ring to the hub.
A brake unit according to the species is known for example from German Patent Application No. DE 297 10 533 U1. The brake unit comprises a ceramic friction ring connected to a wheel hub directly by fastening elements, in other words eliminating a brake disk pot. Such a design of the brake unit makes it possible to reduce the inside diameter of the friction ring considerably and thusxe2x80x94while keeping the external radius essentially the samexe2x80x94to considerably increase the frictional surface area swept by the brake pad. Connecting the friction ring directly to the hub has the additional advantage that a higher run-in accuracy of the brake unit can be achieved at low cost by eliminating intermediate elements. In addition, non-metal friction rings are much more wear-resistant by comparison with metal friction rings and therefore have a much longer service life. A brake unit with a ceramic friction ring mounted directly on the wheel hub therefore combines the advantage of a large brake surface and hence a high braking force with that of a long service life.
However, much higher temperatures develop in friction rings made of ceramic composites than when metal friction rings are used. This results in a high thermal load on the parts located in the vicinity of the brake unit especially when as, in the present case, the friction ring is mounted directly on the wheel hub and therefore no heat-dispersing intermediate part is provided between the friction ring and the hub. In addition, ceramic friction rings are very brittle and hard. If the fastening elements provided on the friction ring and hub, by means of which the friction ring is connected to the hub, have inaccuracies caused by manufacturing, because of its negligible plasticity the friction ring cannot deform in operation so that these inaccuracies are compensated. The result is non-uniform participation of the fastening elements in torque transmission and as a result, more rapid wear of those fastening elements that perform the majority of the torque transmission and are therefore subjected to particularly severe loads during operation.
Hence, a goal of the invention is to improve a brake unit according to the species with a ceramic friction ring connected directly to the wheel hub in such fashion that more uniform torque transmission is ensured between the friction ring and the hub and at the same time considerable heat insulation is ensured between the friction ring and the hub.
The goal is achieved according to the invention by providing a brake unit of the above noted type, wherein the contact surface between the friction ring and the fastening elements is provided with a plastically deformable heat-insulating layer.
The contact surface between the friction ring and the fastening element connected with the hub is provided with a plastically deformable heat-insulating layer. This layer decouples the friction ring thermally from the hub and thus reduces the introduction of heat from the friction ring into the hub. The layer also deforms under load because of its plastic properties so that, during operation, uniform loading of all fastening elements, and hence uniform participation of all fastening elements in the torque transmission between the friction ring and the hub, is produced.
If the friction ring is connected to the hub by a circle of bolts that pass axially through the overlapping edges of the friction ring and the hub, the plastically deformable heat-insulating layer between the friction ring and the bolts is advantageously in the form of sleeves made of a plastically deformable material that deform under load so that uniform loading of all the fastening elements and all the holes in the friction ring is produced. A heat-insulating material is selected as the sleeve material or the sleeves are provided with an additional heat-insulating coating to keep the conduction of heat from the friction ring to the hub as low as possible.
To receive the bolts, the hub advantageously has radial driving slots and the friction ring has matching bores that permit low-load expansion between the hub and the friction ring and at the same time ensure that the friction ring is centered.
In an alternative design, the inside edge of the friction ring is connected with the hub by a spline shaft connection. This offers the advantage over conventional connections of a smaller number of individual parts and thus allows cost reductions. The torque transmission between the hub and the friction ring in this case takes place through the flanks of the spline shaft. The flank profile in this case is chosen so that the friction ring is self-centered on the hub and at the same time a low-force radial expansion of the hub is guaranteed and so that when thermal and mechanical deformations of the hub occur, the forces developing between the hub and the friction ring are as low as possible. The contact surface between the friction ring and the hub is provided with a plastically deformable layer to ensure torque transmission that is as uniform as possible; to minimize the heat transmission between the hub and the friction ring, the contact surface between the hub and the friction ring is made as small as possible and provided with a heat-insulating layer.
In order to ensure a high level of strength for the ceramic friction ring, it is recommended to reinforce the ceramic material by adding fiber material according to certain preferred embodiments of the invention.
Connecting the friction ring directly to the wheel hub increases the frictional surface swept by the brake pad by up to 40% by comparison to conventional connection of the friction ring using an intermediate element. An even more pronounced increase in frictional area can be achieved if two (or more) friction rings are mounted on the hub according to certain preferred embodiments of the invention. During the braking process, the friction rings of two outer brake pads are pressed against a brake lining located between the friction rings according to certain preferred embodiments of the invention. After the braking process, the friction rings are moved apart from one another by a restoring spring so that the middle brake lining runs freely between the two friction rings according to certain preferred embodiments of the invention. By using such a double disk brake with hub connection, the frictional areas can be increased by up to 140% by comparison with conventional brake disks. Another advantage of the double disk brake is the simple design of the friction rings since costly ventilation does not have to be provided.
The brake unit according to the invention is not limited to use in motor vehicles but can also be used in the same way in aircraft and rail vehicles. One important advantage of the brake unit according to the invention consists in the fact that even when the space available for the brake is sharply delimited, a large braking surface is available.